In a wireless communication system, due to the path attenuation of electromagnetic waves, or blockage by buildings, the strength of wireless communication signals may become lower in some areas, and the communication quality of mobile terminals in these areas will be degraded. To solve this problem, relay stations (RSs) are used to enhance the wireless communication signals between base stations (BSs) and mobile stations (MSs). Structures of wireless communication systems having RSs are categorized into two types: the coverage enhanced situation and the rate enhanced situation.
FIG. 1 is a schematic view showing RSs expanding the coverage of a BS. Referring to FIG. 1, MS 1 is directly covered by the BS; however, MS 2 and MS 3 are not directly covered by the BS, but are forwarded by RS 1 and RS 2 to be covered by the BS. The previous situation in which the RSs are used to improve the coverage is referred to as a coverage enhanced situation.
FIG. 2 is a schematic view showing RSs improving the communication rate in the coverage of a BS. As shown in FIG. 2, MS 1, MS 2, and MS 3 are all directly covered by the BS, but MS 2 and MS 3 are distant from the BS. Because of the path attenuation, when the more distant an MS is away from the BS, the lower the strength of the signals that the MS receives from the BS will be. Therefore, the maximum communication rate of an MS is lower if it is more distant from the BS, and the quality of service (QoS) of the MSs distant from the BS is not guaranteed. Therefore, through a higher level code modulation mode, RS 1 and RS 2 enable MS 2 and MS 3 to obtain a rate higher than the rate of direct communication with the BS. The previous situation in which the RSs are used to realize more even communication rate distribution is referred to as the rate enhanced situation.
Irrespective of which wireless communication system structure having RSs is used, to prevent interferences, each of the RSs belonging to a administer area of the same BS always use the time-frequency resources orthogonally when forwarding downlink services of the MSs. In other words, a same time-frequency resource can be used by only one RS. The BS uniformly designates the time-frequency resources used by the RSs. For example, in a downlink service frame of an RS, the time-frequency resource of a burst 3 of RS 1 cannot be used by RS 2, and the time-frequency resource of a burst 4 of RS 2 cannot be used by RS 1 as well. Therefore, in current communication solutions using the RSs, an MS directly communicates with only one RS that has the best communication quality or directly communicates with the BS generally unless the MS is in the handover state.
However, when an MS is covered by a plurality of RSs at the same time (for example, MS 2 in FIG. 1 or FIG. 2), if the distance between the RSs is great, the MS is often at the edge of the coverage of each RS. At this time, normally the BS selects an RS with relatively signal strength to forward the downlink data for the MS. Irrespective which RS is selected to forward the downlink data, due to the relatively large path attenuation, the receiving signal-to-noise ratio of the MS will be low, and the RS will also provide a low communication rate, which cannot satisfy the Quality of Service (QoS) requirements of the MS.